This invention relates to prismatic batteries, and more particularly to safety vents for prismatic battery containers.
Of particular interest in the context of the present invention are rechargeable lithium-ion cells, although other cell types may also benefit from described safety vents. Because of the large amounts of energy stored in lithium ion cells and because of the potentially hazardous nature of some cell components, there is a risk of explosion or uncontrolled release of cell electrolyte.
Lithium-ion cells may operate at pressures in the range of 5-25 pounds per square inch (psi). Such pressures are produced by gases generated during the cell's formation cycle and operation. Cell containers, which are normally made from steel, aluminum or other metal, are designed to contain cell contents at such pressures. Higher pressures can result from overcharging due to a faulty charger, external or internal cell shorting, exposure to excessive heat (e.g., fire), etc. Thus, lithium-ion cell housings should also include some mechanism for controlling the build-up of excess internal pressure. Various safety mechanisms have been devised. One such mechanism is a scored region or vent or reduced material thickness on the cell container. When the cell experiences excess internal pressure the scored region, having relatively low pressure resistance, will rupture and release excess pressure in a controlled manner.
Cell containers come in various sizes and shapes depending upon energy and power requirements as well as the compartment in which the cell will be housed. Cylindrical and prismatic cells are now widely used, although cylindrical cells are more common. Cylindrical cell containers typically have two components: a large cylindrical can and a positive terminal cap. Vents have been provided on both the cylindrical cell can as well as the cap.
Prismatic batteries come in many sizes and geometries, but all have three principal dimensions. Note that cylindrical cells have only two principal dimensions: a length and a diameter. Often, prismatic cells are rectangular in shape. Due to their different shape and construction, prismatic cells and cylindrical cells have different safety design considerations.
Generally lithium ion prismatic cell containers have a large cell can and smaller cell "header." The header portion often contains the terminal for the positive electrode of the prismatic cell, while the can itself serves as the negative terminal and houses the anode, cathode, and electrolyte. In some designs (e.g., those employing aluminum cans), the terminals are reversed. Often the can is a five-sided box structure. Like their cylindrical counterparts, some prismatic cells include safety vents or scoring to release pressure in a controlled manner. For example, one prismatic cell provided by Toshiba Corporation of Japan includes a "dog bone" shaped scored region on the bottom center portion of the prismatic cell can. A different prismatic cell available from Sony Corporation of Japan includes a circular scored region on the "header" portion of the prismatic cell container. The Sony prismatic cell employs a substantially flat header with a slight indentation near its center.
While these designs provide some measure of safety for prismatic cells, they do not appear to have been designed to address certain structural features of prismatic cells. For example, improved designs could be provided in which the manufacture of the vent is made more simply and the vent bursts at a narrowly tailored pressured range.